The invention relates to an audio-frequency powerline carrier control system. Such a powerline carrier control system is known from German Offenlegungsschrift No. 27 50 394.
In powerline carrier control systems, information coded as a train of pulses is sent via a power supply network, which may be three-phase or single-phase, to consumer locations where the signals are decoded and used to control switching operations. For instance, such signals can be used to switch consumer watt-hour meters to different rates or to carry signals to energize warning devices some distance away, such as fire alarms at fire stations. Usually, audio-frequency oscillations in the frequency range between 150 and 500 Hz are used for the duration of predetermined pulse cycles.
For transmission, signals at these frequencies are generated by an inverter and are superimposed on the network frequency by a series-feed circuit. A d-c input voltage impressed on the inverter in part, has a magnitude which is determined by appropriate firing of inverter valves with the audio-frequency transmission signals in the form of voltage blocks of 180.degree. length with alternating sign to the outputs of the inverter. There are two such outputs if the power is transmitted on a single-phase network and three if the network is three-phase. For d-c separation and voltage matching of the phase voltages, it is convenient to connect series inductances and a matching transformer to the outputs. These inductances are made as small as possible, consistent with obtaining good attenuation of the harmonics and not causing excessive voltage drops under load. The series feeding of the transmitted signal is accomplished by current transformers, and if the network is a three-phase one, the primary windings of the transformers are delta-connected and are connected to the phase voltages. The secondary windings of the transformers are connected in series in the respective lines of the supply network. Filter circuits are arranged in parallel with the transformer primaries and are designed as three-phase filters so that they protect the inverter against reactions of the network frequency by presenting a low impedance at the network frequency. On the other hand, they transmit the audio frequency as loss-free as possible to the current transformers by having a high impedance for this audio frequency.
When the transmitter is turned on, i.e., at the start of each transmitting cycle, a considerable switching surge is produced which is not attenuated sufficiently by the small series inductances and which leads to overloading the inverter by large current peaks. It is therefore advantageous, as is proposed in German Auslegeschrift No. 24 56 344, to use an inverter in the form of a pulse inverter in which, by alternately firing the inverter valves, which operate on a respective phase output within each audio-frequency half wave, at a higher frequency, each phase voltage of the inverter output is composed of short voltage blocks of alternating sign. Each phase voltage therefore contains an audio-frequency fundamental and harmonics, and the harmonics are smoothed by the series inductances and possibly also by the matching transformer to that they are fed to the transformer inputs as a nearly sinusoidal voltage, due to resonance processes in the filter circuits. The amplitude of the audio-frequency fundamental can be varied by the duty cycle of the respective inverter valve of a phase output, i.e., by the voltage-time area of the corresponding phase voltage. This makes it possible to increase the pulse inverter to the full output power at the start of each transmitting cycle, by appropriately controlling the duty cycle so that unduly high switching peaks of the current are avoided.
Even so, short peak currents ("overcurrents") can occur at the inverter output due to other disturbances, for instance, load fluctuations, or transients, during which reliable commutation of the inverter is no longer possible. Therefore, the system must be monitored for overcurrent in order to protect the inverter by shutting down the system or by some other intervention into the inverter control. Shutting down should be performed only in extreme emergencies since this can lead only to mutilation of the information and thus to incorrect actions by apparatus or persons receiving that information.
To avoid such an event, the previously mentioned German Offenlegungsschrift No. 27 50 394 includes a monitoring device that determines the phase current between the inverter and the filter circuits and intervenes, if a predetermined current limit is exceeded, into the firing controls of the inverter in such a manner that the amplitude of the audio-frequency phase voltages is reduced by reducing the voltage-time areas to bring the transmitter back to full output power gradually, as in the starting process. It has now been found in some cases that in such a system, in which the firing control of all inverter valves is changed in the direction of reducing the transmitted power if the monitoring system responds, the monitoring system responds in some cases frequently, one time after the other. It is then possible to bring the transmitter to maximum power only very slowly or perhaps not at all. In such a case it it possible that the reduced transmitter power will no longer be sufficient for transmitting the transmitter signals to the receivers.
This can happen, for instance, if the overvoltage peaks are not caused by temporary disturbances, such as would be the case if the filter circuits were short-circuited during the transmitting intervals by corresponding switches opened at the start of a transmitting cycle. Such switches are advantageous to protect the filter circuits during the transmitting breaks.